The importance of copper, as well as a variety of other metals, has led to a continuing search for more efficient and productive procurement methods.
One method of generating high purity copper is pyrometallurgical processing. However, pyrometallurgical processing and purification of metal ores, including copper, inevitably leads to a variety of impurities within the product due to the inherent limitations of the method. Therefore, the initial products must be subject to further purification as a result of the admixed impurities. Additional processes are necessary to bring the crude product to the desired purity standards.
The majority of commercial copper is produced by pyrometallurgical processes, for example smelting. In fact, over seventy percent of the copper metal produced comes from the smelting of copper sulfide concentrates. After the copper is isolated from the copper sulfide concentrates, the copper is cast into copper anodes. These anodes contain a variety of metal impurities that are soluble in the molten copper. At this point, the copper is approximately 99%. However, for some industries, such as wire and electrical operations, the purity of the copper must be at least 99.99%. Moreover, aside from the overall purity of copper, some impurities in particular must be kept to a minimum. For example, bismuth present even in only the ppb can make copper too brittle to pull wire. As with other metals, smelting alone is insufficient to generate high purity copper. Precious metals, nickel, lead, iron, selenium, tellurium, arsenic, antimony, tin, and bismuth are potentially found in this crude copper (“blister”), depending on the ore being treated, and thus further refining is required.
A method to increase the purity of copper is to electrolytically transfer the copper from copper anode sheets to cathode as cathodic copper. As part of the process, the impurities in the anodic copper sheet are dissolved in the electrolyte, or fall to the bottom of the cell as sludge. The build-up of these impurities in the solution causes an issue, as the electrolyte is entrained in the copper cathode sheet as part of the process, thus impregnating the copper sheet with unwanted metal species.
One of the most widely used methods to remove impurities from the “blister” is electrolytic refining. In electrorefining, the blister is re-melted and poured into sheets. These sheets function as the anode of an electrolytic cell, which dissolve and eventually re-plate as the final copper cathodic product. The majority of impurities in the copper anode form an insoluble “slime” on the surface of the electrode or fall to the bottom of the cell. However some impurities, in particular antimony, tin and bismuth, dissolve into the acidic copper electrolyte and can be incorporated into the cathode by numerous mechanisms. These impurities deteriorate cathode quality resulting in concerns for negative downstream processing (e.g., drawing copper wire). Due to the significant economic impact of electrolyte impurities, copper producers go to great lengths to mitigate any and all factors that negatively impact operating costs and/or the quality of the final product.
A common tactic for ensuring a high purity product is to control the concentration of unwanted metals such as bismuth, tin, and antimony in the electrolyte which rapidly increase over time if left unchecked. Typically, the concentrations are maintained at 0.3-0.5 g/L (e.g., bismuth and antimony) in the acidic electrolyte, but when concentrations approach a critical limit, a stream of electrolyte is bled, treated and eventually returned to the cells.
Several methods have been investigated for the removal of the aforementioned impurity metals. Among the variety of methods available for removal of antimony, tin, and bismuth, one option is ion exchange. However, reagent consumption in this method is high, making it a costly and inefficient option. Another option involves precipitation of the unwanted metals. However, this treatment requires altering various characteristics of solution (e.g., acid concentration). This requires additional reagent, and the resulting solution cannot be returned to the electrorefining process due to changes in the solution characteristics (e.g., too acidic). It may also be necessary to recover the copper values before treatment via precipitation. Further options include the electrowinning of the impurities. However this method typically requires the copper be electrowon from the bleed stream and sent back for reprocessing before any other contaminants can be removed from solution. There is thus a need for methods that address one or more of these problems.